(1) Field of the Invention
The present invention relates generally to the manufacture of polyester resins, and to preforms and hot-fill bottles and other containers manufactured therefrom.
(2) Description of the Prior Art
Polyethylene terephthalate (PET) bottles and other containers are widely used for foods and beverages because of their optical clarity, ease of blow molding, gas barrier properties, heat resistance, mechanical strength, and price. To form the resin into containers, the polyester resin is typically first shaped by injection molding into a thick-walled preform, typically in the shape of a tube with a threaded opening, or finish end, at the top. A container is then produced by stretch blow molding the heated preform in a mold having a cavity of the desired container shape. The preform is expanded to fill the mold by rapidly stretching it mechanically in the axial direction while simultaneously forcing air into the heated preform to expand it radially. The resultant containers are acceptable for use in packaging liquids that have a fill temperature at about room temperature.
However, filling of such containers with liquid that is at an elevated temperature, usually as a result of pasteurization or sterilization, tends to soften the container, causing shrinkage and distortion. Accordingly, an additional annealing step is required to heat treat or heat-set containers used in the packaging of heated liquids. Heat setting is normally effected by briefly annealing the container in a heated mold, which may be the same or different from the mold used during the blow-molding step.
For example, hot fill containers may be formed using a heat-setting mold having a temperature of 110xc2x0 C. to 170xc2x0 C., preferably from about 130xc2x0 C. to 150xc2x0 C. A preform is first heated to above its softening temperature, and inserted into the mold. Air is then blown into the interior of the preform with an inserted rod as the preform is elongated. Normally, the air is blown in two stages, a primary or pre-blow stage in which the preform is expanded to the general shape of the container, followed by a secondary or high blow stage at an increased force to ensure that the preform conforms to the interior dimensions of the heated mold. In this secondary stage, the fully expanded container is held against the wall of the heated mold for a brief period, e.g., 0.5 to 0.8 second, so that the polyester undergoes some degree of crystallization, which increases the resin""s thermal stability and reduces the tendency of the container to shrink or distort. Heat setting permits filling of the containers with liquids having a temperature of up to about 100xc2x0 C. without significant shrinkage or distortion.
However, this additional annealing or heat-setting step significantly lengthens the time required to make a container, resulting in reduced productivity and higher costs. Thus, to meet the demands for high-speed production, it is necessary to use a polyester resin that has an increased rate of crystallization. Rapid crystallization of polyester resins, however, tends to result in preforms and containers that have a hazy or cloudy appearance, rendering the containers aesthetically unacceptable for the packaging of liquids for human consumption.
Thus, there is a continuing need for a method of increasing the crystallization rate of polyester resins enabling the rapid production of hot-fill bottles with an acceptable optical clarity. There is similarly a need for a polyester resin suitable to achieve this objective, and for preforms and hot-fill polyester containers exhibiting these properties.